In the electrolytic printing art there are at least two general schemes for printing processes. In one such scheme, metallic ions from one of the electrodes are introduced into the printing sheet and they are either combined with colorless materials already present in the printing sheet in order to form colored complexes or are precipitated as fine metallic particles.
A disadvantage of the above discussed consumable scheme is the fact that the stylus is consumed in the process. This requires complicated printing mechanisms with feeding devices to keep the stylus working.
In another scheme, the electrodes are not consumed and the writing is accomplished by the electrolytic modification of materials already in the printing sheet. An example of such a procedure is one which employs the reaction of starch and iodine to effect writing. Generally, in this scheme, the electrolysis of potassium iodide or another iodide compound in the paper generates free iodine which reacts with the starch which is also present in the paper, thereby producing a purple starch-iodide complex.
Another example of such a scheme includes dry electrolytic printing in which a very special paper is used consisting of one or two metallized layers. Inherent in this scheme are the disadvantages of requiring expensive paper, requiring special layers of materials, and the requirement of voltages that exceed 100 volts for printing.
The nonconsumable schemes, such as the starch-iodine method, suffer from the lack of permanency of the printing due to fading of the printed works and also the discoloration of the paper upon storage.
Another type of electrochromic printing system is disclosed in U.S. Pat. No. 4,211,616 to Sambucetti.
U.S. Pat. No. 4,211,616 is concerned with an electrochromic printing composition which contains an iodide compound as a color former, a bromide, and an auxiliary dye to enhance the color of the printed indicia. As discussed on column 3 thereof, the additional dye is one which would tend to form addition compounds with the iodine and thereby stabilize the printed indicia.
Examples of such auxiliary dyes include leucomethylene blue and derivatives, leuco crystal violet, and 4,4'-methylenebis N-N dimethyl aniline. Suggested leucomethylene blue derivatives include p-sulfonic-benzoyl leucomethylene blue, p-carboxy-benzoyl leucomethylene blue, benzoyl leuco-N,N'p-benzene sulfonic (symmetrical) methylene blue and benzoyl leuco N,N'-p-naptholsulfonic (symmetrical) methylene blue.
Another electrochromic recording substrate is reported in U.S. Pat. No. 4,309,255 to Gendler, et al. which includes a water soluble salt of 3,7-bis(dimethylamino)-10-(2-sulfobenzoyl)-phenothiazine.
U.S. patent application Ser. No. 231,832, now U.S. Pat. No. 4,374,001 to Bernier discloses an electrochromic printing media which comprises a substrate coated with a leucodye having the following formula: ##STR2## wherein A is C.dbd.O or SO.sub.2 ; B is S or O; each R.sub.1 and R.sub.2 individually is a group capable of donating an electron; and R is an organic radical such that in the presence of bromine and upon being subjected to a voltage, the leucodye converts to a colored dye upon splitting off of the A-R group; and coated with a bromide compound to catalyze an electro-oxidation of the leucodye.
Kitakohju, et al. "Dichromic Electrolytic Recording Paper", Fujitsu Scientific & Technical Journal, September 1976, pp. 131-145, suggest an electrolytic printing by the direct electroreaction of benzoyl-leucomethylene blue employing voltages of about 170 or about 230 volts.
U.S. Pat. Nos. 3,772,159; 3,816,838; 3,864,684; 3,871,972; 3,951,757; 3,974,041; 4,012,292; 4,133,933; and Re 29,427 are of interest concerning electrorecording members containing various leucodyes in addition to other required components and the use of very high voltages.
U.S. Pat. Nos. 3,713,996 and 3,726,769 are of interest concerning electrolytic electrosensitive printing.